During routine physical examination of dogs and cats heart murmurs are often noted in the clinical record. Should we worry about this?
Heart murmurs may be associated with a significant cardiac disease or in some cases with diseases of other organ systems such as diseases affecting blood viscosity or cardiac output. Heart murmurs may have a significant impact on cardiac function warranting treatment (‘pathological murmur’), or they may be completely incidental without an underlying disease condition (‘innocent murmur’). Reaching a definitive diagnosis of the cause may benefit the patient, the client, as well as the primary veterinarian – knowing whether treatment is indicated, what is the prognosis and what is the optimal level of monitoring to have a beneficial effect on the long term outcome for the patient as well as reducing the client’s anxiety and concern about their pet. Thus it is always prudent to decide when further investigations are warranted or whether a watchful waiting approach can be safely taken.
The first step in determining the clinical importance of a murmur is careful auscultation. An innocent murmur in dogs and cats is rarely loud (grade III/VI and above) and should not have a diastolic component. Careful auscultation should always be conducted in a quiet environment whilst sufficiently restraining the animal to prevent panting or movement. Auscultation is a skill that can be improved with practice; making a clinical determination on the significance of a murmur requires a sound understanding of the underlying physiology and improves with practice and when combining the auscultation findings with the results of other tests.
Innocent murmurs are more likely to occur in very young (<4 months) animals, they are commonly short and mid-systolic with the point of maximal intensity over the left hemithorax in the region dorsal to the apex beat. These ‘puppy-murmurs’ often change with body position and respiratory phase and their intensity tends to increase with the excitement or exercise. If you are unsure as to the significance of a puppy/kitten murmur, we would are able to provide a brief ‘heart murmur’ consultation to provide reassurance and advice to a worried owner.
As in puppies, systolic murmurs are also more common in adult dogs. Diastolic and continuous (occurring in both systole and diastole) murmurs are invariably pathological and further investigations for an underlying cause is always indicated. When a systolic murmur in an otherwise healthy dog is noted, it is also appropriate to recommend further investigations to clarify the underlying cause and the clinical significance of the murmur. Occasionally identifiable extracardiac disorders may lead to development of murmurs, in which case a thorough cardiac assessment may not be necessary or may be delayed until resolution of the underlying cause. These murmurs are commonly soft and early systolic, with a point of maximal intensity over the left heart base. However, sometimes the diagnostic workup or treatment of non-cardiac conditions requires anaesthesia, intravenous fluid therapy or other medications that might significantly affect the fluid balance in the body (e.g. corticosteroids). In these cases, assessment of the cardiac function may also be recommended prior to these interventions in order to mitigate the risks.
Systolic murmurs are common in clinically healthy adult cats and can be present in 16-44% of routinely auscultated cats. Not all these cats will have a structural heart disease, as the most common non-pathological cause of a murmur in these cats is the dynamic right ventricular outflow tract obstruction. Unfortunately there is a significant overlap in the severity and characteristics of the non-pathological and pathological murmurs in cats which makes it difficult to advise owners on the significance of the murmur without further diagnostic evaluation such as echocardiography. However, the presence of an arrhythmia or gallop sound is very uncommon in a non-pathological murmur and would definitely warrant further assessment as it may indicate an increased risk of a significant underlying cardiac disease, such as hypertrophic cardiomyopathy (HCM) or other forms of cardiomyopathies. Sudden cardiac death and aortic thromboembolisms are associated with cardiomyopathies and an early diagnosis and appropriate management decrease the risk of these severe consequences.
Referral for a full echocardiographic examination is usually required for establishing a final diagnosis and assessing the prognosis, however the following preliminary investigations may often be helpful in determining the presence and significance of a cardiac disease.
Resting respiratory rate at home – the normal resting respiratory rate in cats is <35 breaths per minute. An elevation of the respiratory rate is commonly associated with development of pulmonary oedema.
Blood tests – may help to rule out significant systemic diseases. Especially in geriatric (>10 years old) cats, hyperthyroidism commonly leads to development of murmurs and arrhythmias.
Thoracic radiographs – generalized cardiomegaly, pulmonary venous congestion or abnormal lung pattern are often indicative of a significant heart disease. Also it is worth noting, that in cats the radiographic pattern of pulmonary oedema has a different distribution to dogs and is often ‘patchy’ rather than perihilar. However, sedation of cats with significant ‘silent’ cardiac disease is not without risk, and you may feel more comfortable leaving that to our Specialist Anaesthetists!
Cardiac biomarkers – natriuretic peptides are ‘natural diuretics’ and elevation of the NT-proBNP may suggest elevated cardiac filling pressures, commonly resulting from a structural heart disease. Cardiac troponin I (cTnI) is a biomarker for myocardial damage and its increased concentration is suggestive of myocyte loss or ischemia and thus is commonly associated with severe cardiac disease.
We are currently enrolling cats with asymptomatic HCM in a study looking into nutritional management of HCM. The cats enrolled in this study will be provided with a one year supply of a balanced diet and free exams (including echocardiography) as foreseen by the study protocol. If you see cats with a heart murmur, please do not hesitate to contact us to discuss if they could be eligible for this study. Also, we would be happy to help you with any other patient that has or you suspect might have a heart disease. We have a 24/7 cardiology service including diagnostic and routine screening, interventional procedures and long term follow up care.